Graphite fibers are generally different from carbon fibers in respect of carbon content (purity), fiber structure and fiber characteristics, for example. Graphite fibers are much more useful and effective than carbon fibers when used in sports equipment such as fishing rods and golf club shafts which require high modulus, when used in electric components such as heaters which require high purity and low resistivity and when used for aerospace parts such as aircraft, rockets and the like which require oxidation resistivity and high precision. However, graphite fibers cost much more than carbon fibers and this high cost is largely a result of difficulties in manufacturing processability and productivity. An inert atmosphere is required for production of graphite fibers, and a higher temperature is used than for carbon fibers.
Efforts have been made to increase productivity in manufacturing graphite fibers. For example, it has been proposed to increase the temperature gradient and to shorten the residence time in the graphitizing furnace. However, this produces increased amounts of fuzz on the graphite fiber surfaces and occasionally causes breakage of the running fiber strands.
Also, these modifications tend to reduce the tensile strength of the fibers. Further, since the temperature of the inert atmosphere must be higher than that used for manufacturing carbon fibers, the wear and tear on the graphitizing furnace, particularly on its heating pipes, is very considerable. With such wear and tear due to exceedingly high temperatures, deviations from the desired temperature profile tend to increase very substantially and the furnace tube must be frequently changed. This seriously interferes with productivity and processability, and also consumes large amounts of energy, labor and materials.
With regard to such a graphitizing method, there are one-stage graphitizing methods such as disclosed in U.S. Pat. Nos. 3,700,511, 3,900,556, 3,954,950, 3,764,662 and British Pat. No. 1,215,005.
U.S. Pat. No. 3,700,511 shows a conventional graphitizing method for making a carbon fiber from a fiber which is first oxidized in the temperature range of from 1000.degree. C. to 1600.degree. C. and then successively pyrolyzed up to a temperature of 2500.degree. C. in a graphitizing zone.
U.S. Pat. No. 3,900,556 shows a process for preparing a graphite fiber by rapidly graphitizing an oxidized fiber in a short period of time, such as from 10 seconds to 60 seconds. However, according to this method, it is difficult to obtain as good a temperature profile as possible by this invention. Also, in a rapid graphitizing process, the oxidized fiber is heated very rapidly and develops excessive surface fuzz and tends to break off easily.
U.S. Pat. No. 3,900,556 also shows a rapid graphitizing method. However, the method is in respect to an oxidized fiber, and it uses a carbonizing and one-stage graphitizing procedure. By this method, it is difficult to obtain a small temperature gradient in the vicinity of 1700.degree. C. which is essential in order to obtain a good graphite fiber.
U.S. Pat. No. 3,764,662 discloses a method wherein oxidized fiber is heated at a temperature from 1300.degree. C. to 1800.degree. C. for at least an hour and then a graphite fiber is obtained by heating at a further temperature of from 2300.degree. C. to 3000.degree. C. for 30-90 seconds. However, this method is not practical for an industrial process because of the very long heating time in the first stage. Similarly, the procedure according to British Pat. No. 1,215,005 would not be practical as a commerical process. According to British Pat. No. 1,215,005 a graphite fiber is obtained by successively subjecting an organic fiber, through a first oxidizing furnace to a fourth graphitizing furnace. However, the heat increase rate from the second to the third furnace, which have a temperature range from 1000.degree. C. to 1700.degree. C., is very slow, i.e., 300.degree. C./hr. Also, the residence time in the furnace is very long, namely, from 30 minutes to 4 hours in the second furnace and a maximum of 3 hours in the third furnace. Moreover, the residence time in the temperature range of 2000.degree. C. or more in the fourth graphitizing furnace is also from 30 minutes to 2 hours. Overall, such a process would not be a practical industrial process.
It is an object of this invention to provide a stable method of manufacturing high grade and high quality graphite fibers from carbon fibers made from acrylic fibers, particularly, it is an object to manufacture graphite fibers which have a minimum of surface fuzz. Another object is to provide a process for shortening the heating zone. Another object of this invention is to provide a method of manufacturing graphite fibers in which the matter of changing parts such as furnace tubes and the like, can be easily carried out, wherein the life of the parts is lengthened, costs for energy, materials and labor are materially reduced.
These and other objects are attained by the present process by providing a graphitizing furnace which is divided into two zones and by utilizing specific temperature ranges.